1. Field of the Invention
The present invention relates to arrangements for driving linear motors which move along a linear path of motion, and more particularly to servo arrangements for driving linear motors which move along a linear path of motion and rebound from opposite elastomeric stops.
2. History of the Prior Art
U.S. Pat. No. 4,359,289 of Gordon B. Bararus and Jerry Matula, issued Nov. 16, 1982 and commonly assigned with the present application, describes an arrangement for driving a hammer bank shuttle assembly in reciprocating fashion in a dot matrix line printer. The shuttle assembly forms part of a counterbalanced shuttle drive with a mounting frame therefor being disposed in contact with a spaced-apart pair of rotatable pulleys. A counterbalancing bar is disposed in contact with the pulleys on the opposite sides thereof from the shuttle assembly. The shuttle assembly mounting frame and the counterbalancing bar are held in contact with the pulleys by a flexible band which encircles the pulleys and is fastened to the shuttle assembly mounting frame and the counterbalancing bar and by the magnetic attraction provided by one or more permanent magnets which together with pole pieces are mounted adjacent the shuttle assembly mounting frame and the counterbalancing bar. The permanent magnet provides a flow of magnetic flux through the counterbalancing bar or the shuttle assembly mounting frame or both so as to interact with one or more coils mounted on the counterbalancing bar so as to lie within the flux path. Energization of the coils drives the shuttle assembly in either of opposite directions along a linear path of motion. A pair of stops utilizing springs or other elastomeric elements are positioned so as to be impacted by the shuttle assembly mounting frame or the counterbalancing bar or both when the shuttle assembly reaches opposite limits of movement along the linear path. Each time the arrangement rebounds from one of the stops, the coils are energized so as to drive the shuttle assembly along the linear path of motion at a desired nominal speed.
An alternative arrangement of a counterbalanced bidirectional shuttle drive is shown in U.S. Pat. No. 4,239,403 of Jerry Matula, Glen R. Radke and Gordon B. Barrus, issued Dec. 16, 1980 and commonly assigned with the present application. Such alternative arrangement also utilizes a counterbalancing bar disposed on the opposite sides of a pair of pulleys from the shuttle assembly and opposite elastomeric stops. The arrangement is driven by a motor coupled to rotatably drive one of the pulleys.
As described in U.S. Pat. No. 4,359,289 of Barrus et al, the coils of the electromagnetic motor arrangement are energized by an analog servo which responds at all times to the speed of the shuttle assembly. An encoder is disposed so as to generate pulses in response to the passage of identifiable marks relative to a sensor as the shuttle assembly undergoes motion. The pulses are utilized to determine the actual speed of the shuttle assembly which is in turn compared with a reference and the difference used to vary the drive current to the coils. The drive current as so varied maintains the shuttle assembly at a desired nominal speed as the shuttle assembly sweeps in either of opposite directions through the linear path of motion. When the shuttle assembly reaches the end of its travel in either direction and an elastomeric stop is impacted, the shuttle assembly rapidly decelerates to rest. The servo system senses the rapid deceleration and responds by producing a pulse which tends to drive the arrangement harder into the elastomeric stop. When the arrangement has come to rest and begins accelerating in the opposite direction as it rebounds from the elastomeric stop, the servo again senses the fact that the shuttle assembly is well below the desired nominal speed and responds by generating a pulse in a direction which tends to quickly accelerate the shuttle assembly to the nominal speed in the new direction.
During control of the shuttle assembly in the linear region along the linear path of motion in the arrangement shown in U.S. Pat. No. 4,359,289 of Barrus et al, the analog servo generally remains unsaturated and reacts to the constant measurements of speed by comparing such measurements with the reference and providing the relatively small amount of current typically required to maintain the shuttle assembly moving at the desired nominal speed. During turnaround, however, when the servo senses speeds well below the nominal speed, the servo typically saturates while in the process of generating the large corrective pulses of first one polarity and then the other polarity in the manner described above. This tendency of the servo to saturate, particularly at higher operating speeds where the sudden deceleration and following acceleration of the shuttle assembly constitutes an even greater departure from the desired nominal speed results in a tendency to overdrive the shuttle assembly during turnaround. At the very least the operation during turnaround tends to be inefficient. In more serious cases effective control over the shuttle assembly is actually lost over part or in some cases all of the range of movement of the shuttle assembly.
Accordingly, it would be desirable to provide an improved servo for controlling a linear motor such as one which constantly rebounds at the opposite ends of a linear path of motion. Such servo should be capable of driving the linear motor effectively and efficiently during the rebounding turnarounds as well as during the linear regions of operation.